| 1. |
- Alexander-Webber, J. A., et al.
(författare)
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Giant quantum Hall plateaus generated by charge transfer in epitaxial graphene
- 2016
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322 .- 2045-2322. ; 6
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Tidskriftsartikel (refereegranskat)abstract
- Epitaxial graphene has proven itself to be the best candidate for quantum electrical resistance standards due to its wide quantum Hall plateaus with exceptionally high breakdown currents. However one key underlying mechanism, a magnetic field dependent charge transfer process, is yet to be fully understood. Here we report measurements of the quantum Hall effect in epitaxial graphene showing the widest quantum Hall plateau observed to date extending over 50 T, attributed to an almost linear increase in carrier density with magnetic field. This behaviour is strong evidence for field dependent charge transfer from charge reservoirs with exceptionally high densities of states in close proximity to the graphene. Using a realistic framework of broadened Landau levels we model the densities of donor states and predict the field dependence of charge transfer in excellent agreement with experimental results, thus providing a guide towards engineering epitaxial graphene for applications such as quantum metrology.
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| 2. |
- de Graaf, Sebastian Erik, 1986, et al.
(författare)
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Direct Identification of Dilute Surface Spins on Al2 O3: Origin of Flux Noise in Quantum Circuits
- 2017
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Ingår i: Physical Review Letters. - 1079-7114 .- 0031-9007. ; 118:5, s. 057703-
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Tidskriftsartikel (refereegranskat)abstract
- An on-chip electron spin resonance technique is applied to reveal the nature and origin of surface spins on Al2O3. We measure a spin density of 2.2×1017 spins/m2, attributed to physisorbed atomic hydrogen and S=1/2 electron spin states on the surface. This is direct evidence for the nature of spins responsible for flux noise in quantum circuits, which has been an issue of interest for several decades. Our findings open up a new approach to the identification and controlled reduction of paramagnetic sources of noise and decoherence in superconducting quantum devices.
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| 3. |
- He, Hans, 1989, et al.
(författare)
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Fabrication of graphene quantum hall resistance standard in a cryogen-Table-Top system
- 2016
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Ingår i: 2016 Conference on Precision Electromagnetic Measurements, CPEM 2016; The Westin OttawaOttawa; Canada; 10-15 July 2016. - : Institute of Electrical and Electronics Engineers Inc.. - 9781467391344 ; , s. Art no 7540516-
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Konferensbidrag (refereegranskat)abstract
- We have demonstrated quantum Hall resistance measurements with metrological accuracy in a relatively easy to use and compact cryogen-free system operating at a temperature of around 3.8 K and magnetic field below 5 T. This advance in technology is due to the unique properties of epitaxial graphene on silicon carbide (SiC) which lifts the stringent requirements for quantum hall effect seen in conventional semiconductors. This paper presents the processes involved in fabrication and characterization of metrologically viable epitaxial graphene samples.
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| 4. |
- Huang, J., et al.
(författare)
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Hot carrier relaxation of Dirac fermions in bilayer epitaxial graphene
- 2015
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Ingår i: Journal of Physics Condensed Matter. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 27:16
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Tidskriftsartikel (refereegranskat)abstract
- Energy relaxation of hot Dirac fermions in bilayer epitaxial graphene is experimentally investigated by magnetotransport measurements on Shubnikov-de Haas oscillations and weak localization. The hot-electron energy loss rate is found to follow the predicted Bloch-Gruneisen power-law behaviour of T-4 at carrier temperatures from 1.4K up to similar to 100 K, due to electron-acoustic phonon interactions with a deformation potential coupling constant of 22 eV. A carrier density dependence n(e)(-1.5) in the scaling of the T-4 power law is observed in bilayer graphene, in contrast to the n(e)(-0.5) dependence in monolayer graphene, leading to a crossover in the energy loss rate as a function of carrier density between these two systems. The electron-phonon relaxation time in bilayer graphene is also shown to be strongly carrier density dependent, while it remains constant for a wide range of carrier densities in monolayer graphene. Our results and comparisons between the bilayer and monolayer exhibit a more comprehensive picture of hot carrier dynamics in graphene systems.
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| 5. |
- Huang, J., et al.
(författare)
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Physics of a disordered Dirac point in epitaxial graphene from temperature-dependent magnetotransport measurements
- 2015
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Ingår i: Physical Review B - Condensed Matter and Materials Physics. - : American Physical Society. - 2469-9950 .- 2469-9969 .- 1098-0121 .- 1550-235X. ; 92:7
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Tidskriftsartikel (refereegranskat)abstract
- We report a study of disorder effects on epitaxial graphene in the vicinity of the Dirac point by magnetotransport. Hall effect measurements show that the carrier density increases quadratically with temperature, in good agreement with theoretical predictions which take into account intrinsic thermal excitation combined with electron-hole puddles induced by charged impurities. We deduce disorder strengths in the range 10.2-31.2 meV, depending on the sample treatment. We investigate the scattering mechanisms and estimate the impurity density to be 3.0-9.1x10(10) cm(-2) for our samples. A scattering asymmetry for electrons and holes is observed and is consistent with theoretical calculations for graphene on SiC substrates. We also show that the minimum conductivity increases with increasing disorder strength, in good agreement with quantum-mechanical numerical calculations.
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| 6. |
- Janssen, Tjbm, et al.
(författare)
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Operation of graphene quantum Hall resistance standard in a cryogen-free table-top system
- 2015
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Ingår i: 2D Materials. - : IOP Publishing. - 2053-1583. ; 2:3, s. 035015-
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Tidskriftsartikel (refereegranskat)abstract
- Wedemonstrate quantum Hall resistance measurements with metrological accuracy in a small cryogen-free system operating at a temperature of around 3.8Kand magnetic fields below 5 T. Operating this system requires little experimental knowledge or laboratory infrastructure, thereby greatly advancing the proliferation of primary quantum standards for precision electrical metrology. This significant advance in technology has come about as a result of the unique properties of epitaxial graphene on SiC.
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| 7. |
- Janssen, Tjbm, et al.
(författare)
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Towards a cryogen-free table-top primary resistance standard
- 2016
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Ingår i: 2016 Conference on Precision Electromagnetic Measurements (Cpem 2016). - : Institute of Electrical and Electronics Engineers Inc.. - 9781467391344
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Konferensbidrag (refereegranskat)abstract
- We demonstrate quantum Hall resistance measurements with metrological accuracy in a small cryogen-free system operating at a temperature of around 3.8 K and magnetic fields below 5 T. We use this system to investigate the optimisation of graphene/SiC devices for maximum breakdown current. In addition we report the first characterisation of a cryogen-free cryogenic current comparator which enables entirely cryogen-free primary resistance metrology.
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| 8. |
- Lara Avila, Samuel, 1983, et al.
(författare)
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Influence of Impurity Spin Dynamics on Quantum Transport in Epitaxial Graphene
- 2015
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Ingår i: Physical Review Letters. - : AMER PHYSICAL SOC. - 1079-7114 .- 0031-9007. ; 115:10, s. 106602-
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Tidskriftsartikel (refereegranskat)abstract
- Experimental evidence from both spin-valve and quantum transport measurements points towards unexpectedly fast spin relaxation in graphene. We report magnetotransport studies of epitaxial graphene on SiC in a vector magnetic field showing that spin relaxation, detected using weak-localization analysis, is suppressed by an in-plane magnetic field B-parallel to, and thereby proving that it is caused at least in part by spinful scatterers. A nonmonotonic dependence of the effective decoherence rate on B-parallel to reveals the intricate role of the scatterers' spin dynamics in forming the interference correction to the conductivity, an effect that has gone unnoticed in earlier weak localization studies.
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| 9. |
- Lartsev, Arseniy, 1987, et al.
(författare)
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A prototype of RK/200 quantum Hall array resistance standard on epitaxial graphene
- 2015
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 118:4, s. art nr. 044506-
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Tidskriftsartikel (refereegranskat)abstract
- Epitaxial graphene on silicon carbide is a promising material for the next generation of quantum Hall re-sistance standards. Single Hall bars made of graphene have already surpassed their state-of-the-art GaAsbased counterparts as an RK/2 (RK = h/e^2) standard, showing at least the same precision and higher break-down current density. Compared to single devices, quantum Hall arrays using parallel or series connectionof multiple Hall bars can offer resistance values spanning several orders of magnitude and (in case of parallelconnection) significantly larger measurement currents, but impose strict requirements on uniformity of thematerial. To evaluate the quality of the available material, we have fabricated arrays of 100 Hall bars con-nected in parallel on epitaxial graphene. One out of four devices has shown quantized resistance that matchedthe correct value of RK/200 within the measurement precision of 1e-4 at magnetic fields between 7 and 9Tesla. The defective behaviour of other arrays is attributed mainly to non-uniform doping. This result con-firms the acceptable quality of epitaxial graphene, pointing towards the feasibility of well above 90% yieldof working Hall bars.
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| 10. |
- Shaikhaidarov, R., et al.
(författare)
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Detection of Coherent Terahertz Radiation from a High-Temperature Superconductor Josephson Junction by a Semiconductor Quantum-Dot Detector
- 2016
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Ingår i: Physical Review Applied. - 2331-7019. ; 5:2
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Tidskriftsartikel (refereegranskat)abstract
- We examine the application of Josephson radiation emitters to spectral calibration of single-photon-resolving detectors. Josephson junctions are patterned in a YBCO film on a bicrystal sapphire substrate and are voltage controlled to generate radiation in the frequency range of 0.05-1 THz. The detector used in this work consists of a gate-defined quantum-dot photon-to-charge transducer coupled to a single-electron transistor. Both the emitter and the detector are equipped with a matching on-chip wide-band antenna. The combination of a tuneable emitter and detector allows us to determine the efficacy of the YBCO emitter and also to analyze the elementary processes involved in the detection.
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